It is known that the adhesion capability of plasticized PVC on substrates can be improved by adding, to the plasticized PVC, an adhesion promoter which contains isocyanate groups. This type of improved adhesion capability is important by way of example when the intention is to produce synthetic textile materials provided with a PVC covering. Preference is given to use, as adhesion promoters, of isocyanurates which contain isocyanate groups and which can be prepared by oligomerization, in particular trimerization, from diisocyanates. The diisocyanates most suitable for this purpose are the mixtures, which are readily available commercially, of the isomeric diisocyanatotoluenes (TDI), composed mainly of 2,4-diisocyanatotoluene (2,4-TDI) and 2,6-diisocyanatotoluene (2,6-TDI). These can easily be reacted almost completely to give isocyanurates which contain isocyanate groups. Almost complete reaction is necessary because operator safety and product safety require that the residual content of diisocyanates in the adhesion promoter preparation remains below 1.0% by weight. Diisocyanatodiphenylmethanes (MDI) are likewise readily available but are less suitable, and are more difficult than TDI to trimerize and can therefore lead to undesirably high residual content of diisocyanates. Isocyanurates which contain isocyanate groups based on MDI moreover exhibit poor solubility and have a tendency towards crystallization.
Isocyanurates which contain isocyanate groups are particularly easy to handle as adhesion promoters when they are used in the form of a solution in a plasticizer. In a practical method, the isocyanurates which contain isocyanate groups and are derived from TDI are likewise prepared in the plasticizer used as solvent. DE 24 19 016 A1 (GB 145 570 1A) describes by way of example these adhesion promoters and adhesion promoter preparations comprising plasticizer, and their preparation and their use.
Plasticizers are substances which on mixing with PVC, which is intrinsically hard and brittle, give a soft, tough material known as plasticized PVC. Examples of known plasticizers are the esters of phthalic acid, adipic acid or benzoic acid. Plasticized PVC can comprise large amounts of these plasticizers, sometimes more than 50% by weight of the plasticized PVC. Under service conditions, the plasticizer can separate at the surface or transfer into adjacent materials. The use of plasticized PVC therefore poses the risk of contamination of persons and of the environment by the plasticizer. In the light of these problems, a demand increasingly placed on the plasticizers used is that, with respect to humans, they are harmless and are not bioaccumulative.
According to the European Union Directive 2005/84/EG, the plasticizers di(2-ethylhexyl) phthalate, dibutyl phthalate and benzyl butyl phthalate can no longer be used in toys or baby products, and the plasticizers diisononyl phthalate, diisodecyl phthalate and di-n-octyl phthalate can no longer be used in toys or baby products which can be placed in children's mouths. In view of these restrictions, which many consumers may regard as worrying and difficult to understand, many producers are proceeding towards general elimination of phthalate-containing plasticizers in the production of plasticized PVC. There is therefore a requirement for phthalate-free plasticizers which achieve the performance level of phthalate-containing plasticizers in relation to processability and service properties.
For the purposes of the present invention, phthalate-free plasticizers are plasticizers which comprise no dialkyl phthalate, in particular plasticizers which comprise less than 0.1% by weight of dialkyl phthalate.
Elimination of phthalate-containing plasticizer is also a demand now placed upon adhesion promoter preparations which comprise plasticizer, in particular for sensitive applications, such as toys or baby products. There is therefore a major requirement for adhesion promoter preparations which comprise no phthalates but nevertheless have the good adhesion properties of phthalate-containing adhesion promoter preparations of the prior art. A further demand is that the preparations are clear and free from solids, comprise no volatile solvents and have viscosity smaller than 30 000 mPas at 23° C., for good processability. Residual content of diisocyanates is to be less than 1.0% by weight. There has hitherto been no description of a combination of all of these product properties in the prior art.
For example, the adhesion promoter preparations described in WO 2005 70984 A1 and based on diisononyl phthalate are no longer suitable for sensitive applications. U.S. Pat. No. 4,115,373 A1 and EP 1 378 529 A1 (U.S. Pat. No. 6,936,678 B2) claim that isocyanurates which contain isocyanate groups and which are suitable as adhesion promoters and which are based on TDI can be prepared in any desired solvents, among which are phthalate-free plasticizers. However, the comparative examples set out below show that it is certainly not true that all phthalate-free plasticizers lead to adhesion promoter preparations which meet the requirements described. DE 30 41 732 A1 (U.S. Pat. No. 4,518,729 A) describes, as adhesion promoters, suitable solutions of isocyanurates which contain isocyanate groups but which are prepared from MDI. These solutions are unsuitable, for the abovementioned reasons.
It was therefore an object of the present invention to provide preparations of isocyanurates which contain isocyanate groups and which are suitable as adhesion promoters, where, although the preparations comprise phthalate-free plasticizers, the level of their mechanical properties, e.g. bond strengths, is the same as that of the phthalate-containing adhesion promoter preparations of WO 2005 70984 A1. The isocyanurates which contain isocyanate groups are to be based on industrially available isomer mixtures of TDI, and in particular the use of pure 2,4-TDI is to be avoided for economic reasons. The preparations are to be clear, and their viscosity is to be <30 000 mPas at 23° C., and their content of free TDI (all isomers) is to be <1.0% by weight.